Several interesting things have been said about geolocation research in social media at the 2013 convention of the Association of American Geographers. One of the most surprising is that only slightly more than one percent of Twitter "tweets" are geolocated and 60 percent of that is based off of profile disclosure.
Computer scientist Brent Hecht published a paper in 2011 which investigates this issue in the context of Justin Bieber's fans on Twitter. Hecht discovered 34 percent of the 60 percent of geolocated tweets from profile disclosure are lies. The full paper, Tweets from Justin Bieber’s Heart: The Dynamics of the “Location” Field in User Profiles, is available online. (This makes me now very skeptical of any Twitter-based GIS research)
The abstract of Hecht's paper is below
Little research exists on one of the most common, oldest, and most utilized forms of online social geographic information: the “location” field found in most virtual community user profiles. We performed the first in-depth study of user behavior with regard to the location field in Twitter user profiles. We found that 34% of users did not provide real location information, frequently incorporating fake locations or sarcastic comments that can fool traditional geographic information tools. When users did input their location, they almost never specified it at a scale any more detailed than their city. In order to determine whether or not natural user behaviors have a real effect on the “locatability” of users, we performed a simple machine learning experiment to determine whether we can identify a user’s location by only looking at what that user tweets. We found that a user’s country and state can in fact be determined easily with decent accuracy, indicating that users implicitly reveal location information, with or without realizing it. Implications for location-based services and privacy are discussed.